The present invention relates to a method and apparatus for measuring blood parameters. More particularly, the present invention relates to providing an optical signal proportional to total hematocrit or a concentration of red cells in blood. Still more particularly, the present invention relates to continuous measurement of the vascular blood volume during dialysis.
Many optical sensors for the measurement of whole blood are designed for measuring the light transmission or reflection properties of blood, see, for example, International Application Nos. WO 94/08237 and WO 95/04266; European Patent Nos. 467,804, 800,074 and 818,682; and U.S. Pat. No. 5,351,686.
These measurements often fail to take into account the scattering properties of blood. The omission of this information results in data which is distorted by the light scattering of blood and consequently by properties such as the size, shape and orientation of the cells. The two latter properties are of particular significance when conducting measurements on flowing blood.
A more extensive measurement procedure is required in order to obtain results with minimal contribution of the scattering properties, like an assessment of the angular distribution of the light transmission, as described, for example, in U.S. Pat. No. 5,601,080, by recording the change in transmission at more than one wavelength, as described, for example, U.S. Pat. No. 4,810,090, or at varying sample thickness.
Another drawback with previously known measurement methods for deducing the hematocrit or hemoglobin is that the transmission signal has an exponential relationship with hematocrit or total hemoglobin. This means that the accuracy of the derived results are highly dependent on the quality of the calibration routine used. Moreover, the relationship is dependent on the oxygen saturation level of the blood and the osmolarity. Finally, and also of great importance, is the fact that the relationship is dependent on the flow rate of the blood in a complex manner, making it difficult to correct for the flow rate.
Patients suffering from end stage renal disease are regularly exposed to dialysis, essentially for removing waste products from the blood, balancing electrolytes, supplying buffer and removing excess water. During the removal of excess water, water is removed from the contents of the blood vessels, resulting in a decreased blood volume. The removal is balanced by vascular refilling from the surrounding tissue. However, if the blood volume is caused to decrease too much and too rapidly, the patient may suffer various symptomatic complications, such as hypotension, cramps, nausea and headache.
By measuring the concentration of red blood cells in the blood, a change of the blood volume can be estimated and an excessive reduction of the blood volume may be detected.
One object of the present invention is to provide an optical measurement method and apparatus for measurement of whole blood, which are robust and rugged, and which provide an output signal which is inherently essentially proportional to the concentration of red blood cells.
Another object of the present invention is to provide an optical measurement method and apparatus suitable for measurement of flowing whole blood.
A further object of the present invention is to provide an optical measurement method and apparatus having an increased sensitivity.
A still further object of the present invention is to provide an optical measurement method and apparatus suitable for use as a blood volume sensor during dialysis.
In accordance with the present invention, these and other objects have now been realized by the invention of a method for analyzing the concentration of red blood cells in a flow of blood comprising flowing the blood through a cuvette, directing a light along a direct transmission path through the flow of blood in the cuvette, measuring light transmitted through the flow of blood along the direct transmission path to obtain a transmitted signal, measuring scattered light at a predetermined angle with respect to the direct transmission path to obtain a scattered signal, and providing a ratio signal comprising the ratio between the scattered signal and the transmitted signal. Preferably, the predetermined angle is about 90xc2x0.
In accordance with one embodiment of the method of the present invention, the predetermined angle is between 70xc2x0 and 110xc2x0, and preferably between 80xc2x0 and 100xc2x0.
In accordance with another embodiment of the method of the present invention, the method includes directing the light from a light emitting member, measuring the light transmitted along the direct transmission path by means of a first light sensitive member, and measuring the scattered light by means of a second light sensitive member. In a preferred embodiment, the method includes surrounding at least a portion of the cuvette with light absorbing material in order to increase the sensitivity of the ratio signal. Preferably, the method includes emitting light having a restricted emission angle by means of the light emitting member.
In accordance with one embodiment of the method of present invention, the method includes activating the light emitting member in a pulsed mode.
In accordance with the present invention, these and other objects have also been realized by the invention of apparatus for analyzing the concentration of red blood cells in a flow of blood comprising a cuvette for the flow of blood, a light emitting member for directing a light along a direct transmission path through the flow of blood in the cuvette, a first light sensitive member for measuring light transmitted through the flow of blood along the direct transmission path to obtain a transmitted signal, a second light sensitive member for measuring scattered light at a predetermined angle with respect to the direct transmission path to obtain a scattered signal, and calculation means for providing a ratio signal comprising the ratio between the scattered signal and the transmitted signal. Preferably, the predetermined angle is about 90xc2x0.
In accordance with one embodiment of the apparatus of the present invention, the predetermined angle is between 70xc2x0 and 110xc2x0, and preferably between 80xc2x0 and 100xc2x0.
In accordance with one embodiment of the apparatus of the present invention, the apparatus includes light absorbing material surrounding at least a portion of the cuvette. Preferably, the light emitting member has a restricted emission angle.
In accordance with another embodiment of the apparatus of the present invention, the light emitting member is driven in a pulsed mode.
According to the present invention, it has been observed that both the transmitted signal as well as the side scattered signal have information essential for the evaluation of the red blood cell concentration in blood. Both signals decrease at increasing concentration, and the relationships are non-linear. The signals are dependent on oxygen saturation level, osmolarity and have a complex dependency on blood flow rate.
However, it has also been found, according to the present invention, that the ratio between the perpendicular scattered signal and the transmitted signal is essentially proportional to the red cell concentration, i.e. there is a linear relationship between the ratio signal and the red cell concentration. The ratio signal has, moreover, only a small dependency on the oxygen saturation level, osmolarity and blood flow rate.
Thus, according to the present invention, there is provided a method and apparatus for providing a signal proportional to a concentration of red cells in blood, total hemoglobin or hematocrit, comprising: flowing blood through a cuvette; exposing the flowing blood to light passing along a straight transmission path through the blood in the cuvette; measuring light transmitted along the transmission path to obtain a transmitted signal; measuring scattered light at an angle perpendicular to the transmission path to obtain a scattered signal; and forming a ratio signal which is the ratio between the scattered signal and the transmitted signal. The sensitivity may be increased by adapting light absorbing material at areas surrounding the cuvette or parts thereof.